Ammonia synthesis



Nov. 5, 1968 A. M. FAYoN ET AL. 3,409,397

AMMON IA SYNTHESI S Filed May 19, 1965 REYQQQD twkkm; .Ebb

ADQ LEQQQQ N v A T TORNE Y United States PatentOftlce 3,409,397 AMMONIA SYNTHESIS Abram Miko Fayon, New York, and Jack Barnet Goldstein, Queens, N.Y., assignors to American Cyanamid Company, Stamford, Conn., a corporation of Maine Filed May 19, 1965, Ser. No. 456,947 3 Claims. (Cl. 2.3-199) ABSTRACT OF THE DISCLOSURE Ammonia is produced by reacting hydrogen and nitrogen in the presence of lithium amide at pressures of at least 2400 p.s.i.g. and temperatures of 350-500 C. Under these conditions lithium imide, produced by the ammoniaforming reaction, is converted to lithium amide by reaction with additional hydrogen and nitrogen.

This invention relates to the production of ammonia. More particularly, this invention relates to a continuous process for the production of ammonia from hydrogen and nitrogen.

For commercial acceptance it is desirable that ammonia production be continuous and based on inexpensive starting materials. For these reasons the accepted methods of ammonia production generally involve the direct catalytic conversion of hydrogen and nitrogen to ammonia with reuse of unreacted hydrogen or nitrogen and regeneration or replenishment of the catalyst in a continuous operation.

U.S. Patent 1,737,953 to Urfer discloses a process for the production of ammonia by the reaction of hydrogen, nitrogen and certain nitrogen-containing lithium compounds in the presence of a metal such as iron and an alkali-forming halide. Under these conditions it is taught that conversion to ammonia is achieved at temperatures lower than are ordinarily permissible. The patent is silent in Vregard to whether the nitrogen-containing lithium compound is consumed in the reaction, i.e., Whether it dissociates to form ammonia. The patent is likewise silent as to whether the process could be operated in a continuous fashion to produce ammonia.

Y' A principal object of the present invention is to provide a continuous process for the production of ammonia directly from hydrogen and nitrogen.

A further object of the invention is to provide a continuous process for the production of the ammonia from hydrogen and nitrogen in the presence of a catalytic material which may be regenerated so as to maintain a closed cycle without the necessity of substantially replenishing the catalyst.

These and other objects, advantages and features of the present invention will become more apparent from the detailed description which follows.

In accordance with the present invention there is provided a continuous process for the production of ammonia which comprises reacting hydrogen and ntirogen in the presence of lithium amide wherein the reaction is conducted at a pressure effective for regeneration of the vlithium amide.

3,409,397 Patented Nov. 5, 1968 The sum of Equations 1 and 2 give the overall reaction as follows:

It will be seen that the lithium imide produced as a byproduct in Equation 1 reacts further with the hydrogennitrogen mixture in Equation 2 to regenerate the lithium amide. Further, it will be seen that the lithium amide is not consumed in the overall reaction and therefore provides a catalytic effect. The consequence is a closed cycle and a continuous process.

One of the features which essentially distinguishes the present invention from U.S. Patent 1,737,953, to Urfer is that in order to achieve a closed cycle, i.e., continuous production of ammonia, the present reaction is maintained at or above a critical operating pressure effective for regeneration of the lithium amide in accordance with Equation 2. This critical reaction pressure has been found to be at least about 2400 pounds per square inch gauge (p.s.i.g.). Generally, the higher the pressure above 2400 p.s.i.g. the greater will be the yield of ammonia at a constant temperature However, it is impractical because of prohibitively expensive equipment to maintain the operating pressure above about 4500 p.s.i.g. The preferred operating pressure range is from about 2500 to 3500 p.s.i.g. inclusive.

The reaction temperature must be high enough to promote conversion to ammonia The upper limit is generally about 500 C. since further increases will retard the reaction by favoring the decomposition of ammonia. Thus, effective temperatures are in the range of from about 350 C. to 500 C. inclusive and preferably from about 400 C. to 450 C. inclusive.

The relative amounts of reactants and lithium amide catalyst are not critical although suflicient amounts'of each should be present to maintain reaction rate and product yield at economical levels. A mixing of hydrogen and nitrogen in any proportions will obviously react under the conditions of the invention to provide ammonia. Nevertheless, for eiciency of reaction it is benecial to maintain the mole ratio of hydrogen to nitrogen at about 3 to l, respectively. Preferably the hydrogen will be present in an excess over the 3 moles of hydrogen to one of nitrogen, e.g., 5 to 25% of a mole excess.

The amount of lithium amide catalyst as well as rate of reaction and yield of ammonia will depend on the choice of space velocity of the reaction in addition to operating pressure and temperature. Space velocity is defined as cubic feet of reaction mixture gas corrected to standard temperature and pressure (STP: 0 C. and l atmosphere), passed per hour over one cubic foot of catalyst (solid or liquid). The space velocity is a non-critical operating condition of the invention and may be widely varied as desired, c g., from about 10,000 to 50,000, preferably from about 20,000 to 40,000, reciprocal hours (hn-1) inclusive. The residence time of the reaction mixture is the reciprocal of the space velocity and may be readily calculated from space velocity settings, or vice versa.

The accompanying flow sheet is illustrative of a particular embodiment of the overall process of the invention. In reference thereto, a mixture of hydrogen and nitrogen, preferably in a mole ratio of 3 to 1, respectively, is passed into a gas holder 1 which functions las a reservoir and pressure surge tank for the mixture of the reaction gases. From the `gas holder the gas mixture is compressed to the desired operating pressure in compressor 2, mixed with returnA cycle hot unreacted or product gases as hereinafter described,.and is thenV passe denser 3 operated by a suitable refrigerant to liquefy any ammonia present. Separator -unit 4 thereafter permits the liquid 'ammonia to fa-ll into ammonia receiver 10 and unreacted gases and a minor amount of ammonia to pass into heat exchanger 5 Iwherein the temperature of the gas mixture is raised by heat exchange with effluent hot product vapor from converter 6. From heat exchanger 5 the reactiony mixture moves into the aforementioned con# verter unit 6. Converter 6 is heated by any suitable means such as interior or exterior electric or hot uid jacket, coils or other heat exchange means 0r it may be directly heated with gas 'jets and the like. Converter 6 contains lithium amide in-anylarrangement `effective for maximum contact withthereaction mixture of hydrogen and nitrogen. ,f

The hot product emitted from converter 6 passes through heat exchanger Srvvvherein itis somewhat cooled by heatexchange with incomingreaction mixture. T=hence it enters high temperature condenser 7 in which a suitable coolant such as water, maintained in a jacket or coils interior or exterior-"to the chamber, causes condensation of theV ammonia vapor. The product mixture then passes into separator tank 8 fromwhich the liquid ammonia liows into ammoniareceiver 10. Uncon-densed ammonia and unr'eacted` gas emitted from the separator tank 8 is maintained at the operating pressure `by transmission through compressor 9 -from whereit mixes with incoming reaction mixture gases from v.gas holder 1 to repeat the cycle. The effluent mixture from condenser 9 will, of course, be at a higher reaction temperature than the incoming reaction mixture from gas holder 1 so as to maintain a constantly high reaction temperature throughout the cycle. ,Y c

Although not shown in the ow sheet` it will be apparent that the invention includes a process wherein the catalytic conversion to ammonia corresponding to the reaction of Equation 1 above is conducted in one converter' unit, the ammonia drawn off substantially as herein described, and the lithium amide then regenerated in a second converter in accordance with the reaction of Equation 2 above. While continuity of process may be achieved in this manner no particular advantage is gained thereby, particularly in view of the expense of providing a second .converter unit.

As is known to those skilled in the art it is preferred to avoid.A the presence of moisture in any step or unit of the reaction scheme due to the degradative effect of lwater on the catalyst and equipment. Conventional means will be employed to maintain this substantially dry condition as,

forexample, by employing dry reaction gases, moisture proof lines and units, and the like means.

In addition to the continuity of the process, the invention has the advantage that conventional catalysts employed in the art are unnecessary. For example, the metal catalyst and alkali-forming halide in U.S. Patent 1,737,

95,3 to Urfer need not fbe employed.

The following` examples-illustrate the invention and particularly the criticality of maintainingthe `operating pressure at a level effective for regeneration of the lithium amide. Although the reactions are described as having been conducted in .an autoclave, it will be apparent to those skilled in the art that other procedures such as outlined in the accompanying ow sheet are operable. These examples are .primarily for the purpose of illustration and are not to be construed as a limitation on the invention except as denedin the `appended claims. Unless otherwise ,into low temperature con-Y notedV all partsare by weight andall temperaturesarert degrees centigrade.

EXAMPLES A stainless steel rocking autoclave of 240 ml. capacity, such as described in U.S. Patent 3,155,155, was charged with lithium Iamide under an argon atmosphere. Then was introduced lhydrogen and nitrogen to a predetermined pressure effective to provide the desired mole ratio. The autoclave was heatedunder rocking agitation to obtain the desired. pressure and temperature and maintained under these conditions for the desired reaction time. Thereafter, the autoclave was cooled andthe a'rnount of ammonia 'determined -by` venting the ,autoclave and bubbling the product gases through a' boric acid solution followed by back titration with 0.1 xN .hydrochloric acid solution to give the equivalents of ammonium borate formed. The non-absorbable hydrogen and nitrogen gases were analyzedby mass spectrometry for'ad'ditional information on product distribution. n

The solid residue remaining after each run reachingpr exceeding 350 C. was a hard, gray white iilm which adhered tightly tothe walls of the autoclave. The residue from lower temperature r-uns wasy a powider of the same color. Residue samples from the runs were 'analyzed for lithium, nitrogen and hydrogen.

Tables I and II belowfsummarize test conditions and results for various runs. In Table I, run 3 was conducted using lithium amide and hydrogen only. Some ammonia was formed and the hydrogen consumed approaches a value evidencing the reaction:

However, the formation of the hydride inhibits ammonia production and the yield is'relatively small. The problem was therefore to avoid the formationof lithium hydride.A This was done as reported in Tables I and II using a mixture of hydrogen and nitrogen. Thus runs l and 2 of Table I show significant yields of ammonia. i

In the experiments of Table II conditions for a continuous process were provided by maintaining the original 'charge of lithium amide throughout the entire series of runs and by adding a fresh 3:1 hydrogen-nitrogen Lmixture at the start of'each run. It will'be seen from runs 11 and 12 that no net pressure drop is shown yet am monia at these lower pressures is obtained although lower in yield. This demonstrates that ammonia is not lresulting from dissociation of the lithium amide but rather the lithium amide is catalyzing the conversion of hydrogen and nitrogen to ammonia. If the lithium-amide were dissociating into ammonia a pressure drop corresponding to the resulting ammonia would have been observed. Moreover, the cumulative pressure drop corresponding to 0.422 mole of hydrogen and nitrogen consu-med checks well with the cumulative amount of 0.498 fmole ammonia formed.

It is important to note that in runs l13 and 14 of Table I-I, where the pressure was increased to 2400 p.s.i.g. and 3000 p.s.i.g., respectively, the ammonia yield Ibegan.to rise to the level of runs 1 to 10 and again a pressure drop was observed. This demonstrates that at pressures less than about 2400 p.s.i.g. only the ammonia-producing reaction of Equation 1 above is taking place. Hence, for regeneration of lithium amide for continuous production of ammonia in a closed cycle from a constant streamof hydrogen and nitrogen, the pressure must be maintained at 2400 p.s.i.g. or higher at reaction temperature.

V TABLE 1 Autoclave Charge Maximum Mole I Initial Peak Reaction Percent Run No. LINH: H2 N2 Press. Pres. Temp. .Ammonia Remarks (moles) (moles) y (moles) (p.s.i.g.) (p.s.1.g.) C.) in PGroduct 0.174 0. 913 0.302 1, 600 3, 475 409 3. 8 0.0448 mole NH3 analyzed, reaction time 17 h'rsI 0.174 0.894 0.302 1, 575 3, 550 408 1. 7 0.0197 mole NH3 analyzed, reaction tlme 3 hrs. 0. 174 1. 200 1,600 3, 470 409 1. 2 0.0138 mole NH3 analyzed, reaction time 3 hrs.

TABLE lL-Continuous Synthesis of Ammonia over Lithium Amide l Pres. Pres. Moles Moles Oper. Aver. Running Pres. Moles Moles Moles N H3 Percent Run No. H2 N2 Hz N2 Temp., Oper. Time, Drop, of Gas N H3 per Ammonia C. Pres. hrs. p.s.1.g. Absorbed Mole LiNHz Conversion Totals 0. 422 0. 4982 2. 861

dl The original charge of 0.174 mole of lithium amide was present throughout all the runs. 2 Mass Spectrometer analysis: 77.5% Hz, 22.5% N2, air an H2O free.

We claim: in the presence of lithium amide wherein the reaction is 1. A continuous process lfor the production 0f am- 20 conducted `at apressure of from about -2500 to 3500 p.s.i.g. mOIli Which comprises reacting nitrogen and hydrogen and a temperature of from about 400 C. to 450 C. in the presence of lithiu-m amide at a pressure of at least 2400 p.s.i.g. and a temperature in the range of 350 C. References Cited to 500 C. said pressure and temperature being effective 2 UNITED STATES PATENTS for regeneration of the lithium amide. 5

2. A process according to claim 1 wherein the space 1159364 11/1915 De Jahn 23-198 X velocity of the reaction mixture gas is within the range of 20,000 to 40,000 reciprocal hours. OSCAR R' VERTIZ Pnmary Exammer' 3. A continuous process for the production of am- H. S. MILLER, Assstant Examiner.

monia which comprises reacting hydrogen and nitrogen 30 

